EUV light, e.g., electromagnetic radiation having wavelengths of around 50 nm or less (also sometimes referred to as soft x-rays), and including light at a wavelength of about 13.5 nm, can be used in photolithography processes to produce extremely small features in substrates such as silicon wafers. Here and elsewhere, it will be understood that the term “light” is used to encompass electromagnetic radiation outside of the visible part of the spectrum.
Methods for generating EUV light include converting a source material from a liquid state into a plasma state. The source material preferably includes at least one element, e.g., xenon, lithium or tin, with one or more emission lines in the EUV range. In one such method, often termed laser produced plasma (“LPP”), the required plasma can be produced by using a laser beam to irradiate a source material having the required line-emitting element.
One LPP technique involves generating a stream of source material droplets and irradiating at least some of the droplets with laser light pulses. In more theoretical terms, LPP light sources generate EUV radiation by depositing laser energy into a source material having at least one EUV emitting element, such as xenon (Xe), tin (Sn), or lithium (Li), creating a highly ionized plasma with electron temperatures of several 10's of eV.
The energetic radiation generated during de-excitation and recombination of these ions is emitted from the plasma in all directions. In one common arrangement, a near-normal-incidence mirror (often termed a “collector mirror” or simply a “collector”) is positioned to collect, direct (and in some arrangements, focus) the light to an intermediate location. The collected light may then be relayed from the intermediate location to a set of scanner optics and ultimately to a wafer.
In the EUV portion of the spectrum reflective optics are usually used for the collector. At the wavelengths involved, the collector is advantageously implemented as a multi-layer mirror (“MLM”). As its name implies, this MLM is generally made up of alternating layers of material over a foundation or substrate.
The optical element must be placed within the vacuum chamber with the plasma to collect and redirect the EUV light. The environment within the chamber is inimical to the optical element and so limits its useful lifetime, for example, by reducing its reflectivity. An optical element within the environment may be exposed to high energy ions or particles of source material. The particles of source material can contaminate the optical element's exposed surface. Particles of source material can also cause physical damage and localized heating of the MLM surface. The source materials may be particularly reactive with a material making up at least one layer of the MLM, e.g., molybdenum and silicon. Temperature stability, ion-implantation and diffusion problems may need to be addressed even with less reactive source materials, e.g., tin, indium, or xenon.
Several techniques have been employed to increase optical element lifetime despite these harsh conditions. For example, protective layers or intermediate diffusion barrier layers may be used to isolate the MLM layers from the environment. The collector may be heated to an elevated temperature of, e.g., up to 500° C., to evaporate debris from its surface. The collector surface may be cleaned using hydrogen radicals. An etchant may be employed e.g., a halogen etchant, to etch debris from the collector surfaces and create a shielding plasma in the vicinity of the reflector surfaces.
Another technique which may be employed is to reduce the likelihood that contaminating source material reaches the collector surface. Source material may accumulate on the interior surfaces of the vessel. This source material may reach the collector through the influence of gravity. There is a need to protect the system from this material. For example, some systems use vanes to protect the collector from micro-droplets of source material created during plasma generation. It is possible in such a system, however, for source material to accumulate on the vanes. This in turn creates the possibility that accumulated source material will detach from the vanes and impinge on the surface of the collector, especially when the system for dispensing source material into the vessel is deployed at a large angle from vertical.
There remains a need to extend collector lifetime by protecting the surfaces of optical elements from source material in systems for generating EUV light. With this in mind, applicants disclose arrangements for improved protection of surfaces of optical elements.